1. Field of the Invention
This invention relates in general to an improved startup procedure on ammonia plants for the synthesis of ammonia. Such a process is beneficial for providing a time savings, a cost savings and an energy savings. In particular, this new procedure comprises the simultaneous preheating of the primary and secondary reformers with steam while also preheating a low temperature CO shifter with preheated desulfurized hydrocarbon gas. After these systems have reached a satisfactory temperature, feed streams of preheated desulfurized hydrocarbon gas and steam are injected into the reformers along with air to the secondary reformer to produce reformed gas which is then used to preheat the high temperature CO shifter. Simultaneously, exhaust gas from the low temperature CO shifter along with a portion of the gas converted in the high temperature CO shifter is used to preheat the CO.sub.2 removal system. Once these systems have been heated sufficiently, the preheated feed steams from the primary and secondary reformers are passed through the high temperature CO shifter, through the low temperature CO shifter, and through the CO.sub.2 removal system into a methanator in order to preheat it. From the methanator, the feed streams are passed into a preheated synthesis converter system wherein a converter vessel is heated to start the production of ammonia.
2. Description of the Prior Art
The principal industrial process developed for the production of ammonia involves the use of hydrocarbon feedstocks to furnish the source of hydrogen e.g., natural gas, for the reaction with atmospheric nitrogen to form ammonia. In such ammonia synthesis trains, the final production cost is affected by the length of time it takes to bring the train on stream and the amount of hydrocarbon gas feed which is required to preheat the various systems. Both of these factors are directly affected by the startup procedure one uses.
In recent years, there has been an increasing concentration on constructing ammonia plants on a larger scale, those have a capacity of from 600 to 1,500 tons/day. In addition, most of these new plants are designed to handle a natural gas feedstock, instead of coal or oil, and the price of natural gas has been rapidly increasing due to the world's current energy problem. These larger plants require a longer startup time because the equipment is proportionally larger and therefore it takes a greater amount of time to accomplish the preheating phase. A typical ammonia plant will, on the average, be completely shut down and started up again three to five times a year with each startup requiring approximately forty-sixty hours. These frequent shut downs are necessitated by mechanical equipment failures and routine turn-a-rounds.
The most common causes of delays in startup are attributed to: startup operating conditions, equipment problems and operator's human error. Included in start-up operating conditions are difficulties more severe than those for which the equipment is specified, a typical example is the sequential double contingency such as trip out of a large turbine followed by mal-operation of the automatic turbine bypass valve. The resulting imbalance to the steam system requires shutdown and subsequent delay.
Under equipment problems are included a large variety of problems with one common feature--the equipment has not been manufactured or fabricated in accordance with specifications and drawings. The final category of human errors include all incidents where delay is attributed to operator error or improper direction and supervision. Although some of these delays in startup cannot be anticipated or avoided, it has been found that our improved process will reduce the time during which the plant is not on-stream. This is significant in terms of economics for anyone familiar with ammonia plants realizes that there is a strong incentive to get on line as quickly as possible. A standard cost estimate for a 1,500 ton on stream ammonia plant is $40,000/day, and when the plant is not on stream, the income is zero. By practicing our improved process one is able to decrease startup time by 10-20 hours depending on plant design. This reduction in time multiplied by the cost of natural gas, for example $2/million cubic feet, can result in a substantial cost saving. In addition, when an ammonia plant is being brought on stream, steam and hydrocarbon gas are used to bring the various systems up to the desired temperature. In most cases, the fumes emitted from the burning of this gas with air contains CO, CO.sub.2, and nitrogen oxides, and these fumes are usually vented directly to the atmosphere. By reducing the time of startup one is able to reduce the amount of undesirable particulates which will enter the atmosphere. Therefore, a reduction in startup time has an additional environmental impact.
U.S. Pat. No. 3,947,551 of Parrish discloses an improved ammonia synthesis train utilizing hydrocarbon starting materials which are converted to hydrogen under superatmospheric pressure in a series of steps which include conversion of the hydrocarbons to hydrogen-carbon monoxide mixtures, shift conversion of the carbon monoxide to hydrogen and CO.sub.2, and removal of the CO.sub.2. In such a system, reduction in process gas losses is achieved through an improved arrangement and integration of the shift conversion and CO.sub.2 removal steps. However, this patent does not teach the simultaneous preheating of certain vessels and/or systems to reduce startup time.
The general object of this invention is to provide an improved startup procedure for an ammonia plant so as to reduce startup time and conserve energy. A more specific object of this invention is to provide a new process which uses less hydrocarbon gas to preheat the various systems, thereby contributing to a cost savings.
Another object of this invention is to provide an improved startup procedure for an ammonia plant which reduces the amount of noxious emissions into the atomosphere.